Broadband communications access, on which our society and economy is growing increasingly dependent, is becoming pervasive in all aspects of daily societal functions. For example, broadband communication has become increasingly available to users on board mobile platforms such as aircraft, ships, automobiles and trains. Broadband communication services for passengers of mobile platforms include Internet access, e.g., e-mail and web browsing, live television, voice services, virtual private network access and other interactive and real time services. While the technology exists to deliver broadband communication services to effectively all types user terminals, e.g., mobile platforms, affordable delivery of these services has been a challenge for communication systems that serve a wide range of communication terminals having wide ranging capability and operating in a wide range of time varying link conditions. The problem is particularly acute for mobile communication terminals that must often be smaller than convention fixed terminals and must operate under a greater range of conditions.
Broadband communication systems for remote, hard to access, or mobile user terminals, e.g., mobile platforms, often use communication satellites that can provide service coverage over large footprints, often including remote land regions and oceans For such satellite communications systems that employ geosynchronous satellites, the footprint often covers a relatively fixed region of the earth. For satellite communications systems that employ satellites with low and medium earth orbits, the footprints cover a moving region, in other words the footprints sweep across the earth. Generally, base stations, e.g., a ground based station, send data and information to the user terminals through a bent pipe via one or more satellites. More specifically, the base stations send data information on a forward link to the satellite transponder that receives, amplifies and re-transmits the data and information to an antenna of one or more user terminals, e.g., fixed locations on the earth or one or more mobile platforms such as aircraft, ships, trucks, trains, etc. The user terminals, in turn, can send data back to the base stations via the satellite transponder. The base stations can provide the user terminals with links to the Internet, public switched telephone networks, and/or other public or private networks, servers and services.
In many applications in which maximum efficiency is required, the forward link from the base station to the user terminals of communication systems is commonly operated with a single carrier so that the power amplifier in the transmitter can operate at the maximum saturated power level without significant degradation due to nonlinear mixing of multiple carriers. In satellite communication systems, maximum efficiency is achieved when the transponder operates at its saturated output power level. That is, the satellite communication systems are operated with little or no transponder output back-off. Typically, single carrier operations utilize time division multiplexing access (TDMA) to share the communication channel between multiple receiving user terminals in the coverage region for the communication system. All data and information destined for user terminals within the transponder coverage region is typically transmitted using a waveform, having FEC coding, modulation, and information rate, that can be successfully received by all user terminals in the coverage region. The problem with this type of operation is the inefficiency that occurs when there is a wide range of user terminal antenna sizes, loss conditions, e.g., rain fade, and satellite downlink effective isotropic radiated power (EIRP) variation within the coverage region.
In satellite communication systems of this type, the forward link waveform is selected to “close” the communication link with the most “disadvantaged” user terminal. That is, the forward link waveform is selected to establish a successful communication link between the base station and the user terminal having the smallest antenna size, highest rain fade and/or, lowest satellite downlink EIRP location. Therefore, the most “advantaged” user terminals, i.e., the user terminals having the largest antenna size, no rain fade and/or a high satellite EIRP location, operate with excess margin. In many instances, the dynamic range between the most disadvantaged and most advantaged user terminals can be greater than a factor of 100 (20 dB), which can result in large inefficiency when conveying unicast and/or multicast traffic. Thus, the excess margin available to advantaged user terminals that could be converted into higher information rates to reduce the cost per bit to deliver information to the user terminals is unused.
Adaptive coding and modulation (ACM) has sometimes been utilized in attempts to address the problems of such “one size fits all” approaches. ACM dynamically adjusts forward error correction coding (FEC), often referred to as simply “coding”, and signal modulation to adapt to the conditions of the communications link to each individual terminal. Thus, a disadvantaged user terminal is sent information with a different coding rate and modulation order than an advantaged terminal. Despite the potential performance improvements provided by ACM, the range of terminals and link conditions over which these performance gains can be achieved is limited. For example, the range of user terminal antenna aperture sizes at which coding and modulation changes are effective at adapting to link conditions is mostly limited to standard sized VSATs (very small aperture terminals). Generally, in the United States, VSATs are very common throughout the U.S. and the world with hundreds of thousands of the terminals deployed in fixed locations such as gas stations, banks, etc. These terminals generally have apertures greater than 0.8 meters in diameter at Ku-band and 3 meters at C-band. Thus, ACM is not useful for a wide range of “smaller-than-VSAT sized terminals”, which are commonly used for satellite communication to mobile users terminals. For example, ACM is not useful for most user terminals on board mobile platforms, such as aircraft, trucks, automobiles, boats, trains, etc., that cannot accommodate a full-sized VSAT.
Additionally, with ACM, as a user terminal of any size becomes more and more disadvantaged, due to deteriorating link conditions, the receive Eb/No becomes smaller and smaller. In response, the ACM continuously attempts to decrease the threshold Eb/No to maintain a positive link margin by reducing the code rate and by reducing the order of modulation. Generally, margin is defined as the difference between the receive Eb/No and the threshold Eb/No, and excess margin is defined as margin greater than a prudent safety margin for unknown losses and variations in the link. For example, the excess margin is typically less than 2 dB. Eventually, the threshold Eb/No can not be further reduced by further reduction of the coding rate and the order of modulation. At this point, the ACM concept breaks down because no amount of additional reduction in code rate or order of modulation can be applied to reduce the threshold Eb/No in order to maintain a positive margin.
Furthermore, as exemplarily illustrated in FIGS. 1 and 2, most practical systems that do not employ spread spectrum modulation have a symbol rate Rs that is approximately equal to an occupied signal bandwidth (W). Once the modulation order and code rate are set to their limits for minimizing threshold Eb/No and maximizing power efficiency, i.e., they are set to constant values, then the symbol rate Rs is approximately equal to the information rate Ri. Therefore, changes in information rate Ri to adapt to different link conditions necessarily create proportional changes in the signal bandwidth W. Accordingly, it is desirable to keep the occupied signal bandwidth constant as the information rate Ri is changed to adapt to link conditions. Therefore, conventional (non-spread-spectrum) methods are not suitable. For example, the relationship between threshold Eb/No and code rate for a particular forward error correction type called low density parity check (LDPC) is shown in FIG. 1 (assuming QPSK modulation). As illustrated, the threshold Eb/No decreases as the code rate decreases until a minimum is reached at a code rate=1/3. A similar example showing the reduction of threshold Eb/No with order of modulation, is shown in FIG. 2. As illustrated, the minimum threshold Eb/No occurs when for a code rate=1/3 and modulation order=2 (QPSK modulation). Once the code rate and the modulation order reach these values, a positive margin can no be maintained to adapt to a fading condition on the link by further reducing the code rate and modulation order, Therefore, the occupied signal bandwidth W can not be held constant.